CC'ing chambers

[170-3tree]

I'm not sure if anyone has asked this or if this is the right spot to ask, (tried search no avail) but I'm prepping to do a bit of porting and cc'ing on my chambers as well as maybe a cam, etc. I Know that the log head naturally causes uneven a/f ratio in our engines, but is there a way to vary the cc's or something to minimalize this effect. Like making 1,3,4,6 either larger or smaller to fool the engine into a more equal a/f ratio.
Any input or a link would be awesome. Thanks.

 

[bubba22349]

Most people will use a later 77 or up head these have the largest logs.

 

[170-3tree]

I know, but I'm trying to build a strictly drivability engine, a good even combination of efficiency and power. This head has a better design chamber for burn, from what other members have told me. I want to increase air velocities into and out of the engine and the smaller log will allow that more. the only downside it gives is that I would have to be a magician to properly mount a Webber 32/36 so I can have that smaller venturi-primary.

 

[CZLN6]

Howdy back 170:

I'm assumming that you are working with a kidney-bean shaped combustion chambered head. wat is the casting number? What carb/ignition will you be using? the 170 engine does not have dished pistons, but decking the block to zero as a way to maximise CR will help and maintain a stock or higher CR.

Ironically, some of the best things you can do to help even out a/f distribution are not with the head. AKT what makes for a more efficient distribution at lower rpms are not the same at higher rpms, and vise-versa. Your goal will be to maximize vacuum and compression ratio.

A clean, lean A/F mixture is number one. Maintaining high velocity/high vacuum is also right up there. So higher compression and an efficient valve and valve seat area are critical- not larger valves. Intake is drawn in by vacuum, is wet and being pumped along, so rounding right angle turns helps. I would strive to maximise these issues rather than trying to create lumps or vanes, which will disrupt flow and velocity. Limit your porting to smoothing the transition from machined to cast in the valve pockets, specify at least a three angle valve seat and have the intake valves back cut with a 30 degree angle above the stock 45 degree valve angle. Do not be tempted to open the combustion chamber. You should strive to maintain maximum chamber-to-bore ratio for best quench effect.

Assuming a good ignition system you can fine tune individual cylinders by adjusting the heat range of the spark plugs. This is commonly done to help weak cylinders with oil fouling, but the concept is the same but in reverse. richer cylinders get a slightly hotter plug, leaner cylinders get a slighty cooler heat range. This in indeed fine tuning, and is not likely to effect performance.

Keep us posted on your thoughts and progress.

Adios, David

 

[170-3tree]

Thank you David,

The end result of this project will include a DUI ignition and for now a pony carb to get me by. In the long run I might go to either one or three pinto throttle bodies, depending on what I think I want to pay. It will also have headers with dual exhaust (size is undecided as of now)
I am not willing to pull the entire engine at this time as it only has 11k on the rebuild so decking the block is not likely.
I plan on doing the porting myself. Knocking down lumps and burrs while smoothing transistions. And I will likely take the head to get valve seat work done as well. I don't plan on making any HUGE changes just simplifying air paths to the chamber a bit. I wasn't going to make huge chambers either, but making sure they are equal in size and smoothed out would likely be beneficial. (I'm fully aware most of this will not shave seconds off my time or add 10 mpg's, but I like to tinker with my car and port work like this is FAR less expensive than anything else I can throw into it) Ideally I want to also find a way to improve atomization, but that will likely come with making and modifying a phenolic spacer to replace the carb-heater plate there now.